A typical computer system includes an input/output controller hub (“ICH”). The ICH has a number of ports, via which other components within the computer system are coupled to the ICH. The ports include signal drivers to send signals to the other components. The slew rate of the drivers has to be precise in order to accurately drive the edge of an output signal.
To achieve a precise slew rate in driving the output signal, the current implementation of an output port uses an output driver similar to a digital-to-analog converter (“DAC”), hereinafter referred to as a DAC-like driver, to precisely shift the analog output level of the differential data lines in a stair-step fashion from rail to rail. To generate an output stair step waveform with seven steps, seven identical DAC-like drivers are needed to drive one output signal. The seven identical drivers receive the outgoing data signal passed along seven shift registers. Each shift register is coupled to the input of one of the DAC-like driver. As the data signal from the core of the ICH propagates through the shift registers, each DAC-like driver is successively turned on. The resultant waveform looks like a stair step with transitions in seven equal steps. A DAC-like driver drives the D+ data line and another DAC-like driver drives the D− data line. The D+ and D− data lines are driven opposite to one another.
Driving the data lines in the stair step waveform allows effective control of the output rise time and fall time because each step is identical to one another. Furthermore, the signal cross-over point of the D+ and D− data lines can be easily positioned within one period of the transmit clock by aligning the steps in the middle of the D+ waveform and the D− waveform. The transmit clock cycle and the number of steps may be selected in order to provide the required signal slew rate.
However, the DAC-like driver is a space-consuming design. To worsen the problem, the number of DAC-like drivers required is equal to the number of steps desired in the output stair step waveform. Therefore, multiple DAC-like drivers are needed to generate a single stair step waveform, further complicating the design of the output port.